Torsional mode transducer systems have been described extensively in a book entitled “Sources of High-intensity Ultrasound,” Volume 2, and more specifically in Part IV, which is entitled “Torsional Mode Vibration Systems,” written by A. M. Mitskevich and edited by Rozenberg in 1969. FIG. 1 illustrates one type of a torsional mode system disclosed therein. The system illustrated in FIG. 1 is normally used for welding, for example, in specialist applications such as the helium tight sealing of cans and containers. Magnetostrictive vibrators with longitudinal waveguides 101 are attached to a rod 102 with an end mass 103, wherein they excite torsional vibrations, which are transmitted to the welded parts 104 situated on the supporting platform 105. Various known modifications to this system include the replacement of the magnetostrictive vibrators with more efficient piezo-electric vibrators and the use of two vibrators in a push-pull mode. Mitskevich concludes that the system illustrated in FIG. 1 is awkward, inconvenient and extremely unsuitable from the energy point of view.
Torsional mode transducer systems that include an end effector for surgical applications, specifically for cutting and coagulating tissue have been described by Young (U.S. Pat. No. 6,425,906). The transducer system disclosed by Young is illustrated in FIG. 2. Young attempted to eliminate longitudinal motion by attaching the longitudinal transducer 202 at right angles to the torsional mode waveguide 204. The motive force for transducer 202 is provided by piezo electric drive elements 203. Young noted that the use of torsional mode vibration for ultrasonic scalpel/coagulation applications is safer because energy is absorbed into the target tissue and not transmitted along the waveguide axis into distant regions. One disadvantage of this design geometry is that it is difficult to incorporate within a slim ergonomic surgical tool that is both compact and light weight.
In addition to torsional mode transducer systems, there are longitudinal-torsional (L-T) mode transducer systems. These L-T mode transducer systems are rod systems, which, when driven in a longitudinal mode, are capable of generating a torsional vibration component by virtue of a certain inhomogeneity in the cross section of the rod. Mitskevich (cited above) has described such systems. One such device consisted of an ultrasonic horn 300, as is shown in FIG. 3. The horn, itself, is marked with gradually deepening grooves 303; these form a helix with a smooth diminishing pitch. Excitation over the frequency range 15 kHz to 21 kHz was accomplished by means of a ferrite or magnetostrictive transducer (not shown) attached by the screwed thread 301 at the proximal end of the horn. The variation in the tangential (x) and longitudinal (y) components of vibration at the distal tip of the horn 302 as a function of driving frequency is shown in FIG. 4. As can be seen in FIG. 4, the longitudinal component (y) at the distal tip of the horn 302 is reduced to zero at a frequency of 16.5 kHz resulting in a single tangential mode of vibration. FIG. 4 also shows that the tangential or torsional mode of vibration is reduced to zero at a frequency of approximately 17.8 kHz resulting in a single longitudinal mode of vibration. Additionally, the tip of the horn 302 vibrates in a combined L-T mode at frequencies other than 16.5 kHz and 17.8 kHz (see FIG. 4). For example, at a frequency of approximately 16.3 kHz the component of longitudinal vibration is similar to the component of tangential vibration. Mitskevich also describes L-T resonators made by creating an inhomogeneous cross section along the length of an otherwise uniform bar and then twisting the bar along its length. The same structure can be obtained by using a conventional twist drill or by machining the grooves into the bar.
Wuchinich (U.S. Pat. No. 6,984,220) disclosed the design of a similar longitudinal-torsional device that operates at a combined L-T resonance and is used to dissect biological tissue. The transducer and L-T resonator system 500 disclosed by Wuchinich is reproduced in FIG. 5. The motive force for transducer 519 can be either magnetostrictive or piezoelectric and is designed to operate as a half-wave resonator. The longitudinal vibrations 523 at the distal tip of the transducer are coupled to resonator section 521 that has an inhomogeneous cross section that converts the single longitudinal motion into a combined L-T motion at the tissue contacting tip 524. The inhomogeneous cross section can be in the form of a helical spiral spring similar to that illustrated in FIG. 3.
Use of the Wuchinich design for ultrasonic handpieces used for surgical procedures such as cataract removal (phacoemulsification) and dental teeth cleaning would result in suboptimal handpiece in terms of length and weight. Typically, these handpieces operate at frequencies>28 kHz and <40 kHz. Operating above 28 kHz reduces the risk of an audible sub-harmonic frequency and operating below 40 kHz optimizes the design for maximum displacement of the end effector at the operative site. The maximum operational frequency for a medical handpiece is about 250 kHz. Designing a 28 kHz piezoelectric transducer/L-T resonator using the teachings of Wuchinich would result in a handpiece design that would have an overall length of about 200 mm (8 inches) if allowance is made for electrical connection at the proximal end of the transducer. This length is significantly longer than existing current designs and would be heavier, thus making it impractical to use for these applications.
Boukhny (U.S. Pat. No. 6,077,285) also described an apparatus for providing both longitudinal and torsional ultrasonic motion for the purpose of enhancing tissue dissection. His device utilizes separate torsional and longitudinal transducers systems to provide this motion. To obtain the desired result requires the simultaneous operation of both transducer systems. To supply the power required the use of two electrical generators, one for each of the different transducer systems. Furthermore, all such devices as described by Boukhny, whether longitudinal, transverse or torsional must be fixed within an enclosure, such as a handpiece, preferably at points where there is no motion, known as motional nodes. However, because the wavelength of torsional and longitudinal vibration is substantially different, the node or nodes for longitudinal vibration and torsional motion will be located at different points on the transducer system and other portions of other resonators attached to the transducer system. Hence, no true motionless point may be found. The result being that either longitudinal or torsional motion will be communicated to the handpiece and thereby to the operator holding the handpiece. Although, vibration isolators can be utilized to prevent the communication of such unintended motion, if they are truly isolating they invariably complicate construction of the device and, if simple, consume power in the form of heat generated by contact with a moving surface. Hence, Boukhny's device is both complicated to operate, needing two separate power sources, and is difficult to construct.
Although the magnetostrictive transducers have been replaced by more efficient piezo-electric transducers, the coupling of energy into the torsional mode is much lower than the coupling of energy into the longitudinal mode. Typical measured values of effective coupling coefficient for torsional mode are between 0.04 and 0.08 whereas the effective coupling of longitudinal mode is typically >0.1. FIG. 4 shows a damped torsional mode characteristic (x) compared with the longitudinal mode (y). This results in significantly higher value of electrical impedance that typically has a large reactive component. This can present a system control problem and the high operating voltage limits the torsional mode power that can be delivered to the operative site.
Therefore, as to these L-T transducer systems, Rozenberg in “Sources of High-intensity Ultrasound,” Volume 2 concludes that “despite the number of obvious advantages of Longitudinal-Torsional mode (L-T) systems, they have not been put to use on a sufficient scale. One of the main reasons for this is a lack of at least an approximate method for the calculation of such systems” This problem is compounded because the experimental optimization process is complex and involves the fabrication of a large number of sample L-T waveguides.
For reasons stated above, there is a need for optimized ultrasonic transducers that provide torsional modes of motion and/or L-T modes of motion. In particular, there is a need for small, uniaxial, light weight relatively low power torsional and L-T handpieces for medical applications including phacoemulsification applications and dental applications, such as for example, but not limited to, teeth cleaning and tooth extraction. Additionally, there is a need for higher power L-T transducer systems for industrial applications and also medical orthopedic applications such as bone cutting. The invention described herein addresses these and other needs.